Could anyone provide recommendations on the most effective strategy for expanding my real estate portfolio?

I'd recommend talking to an attorney.  

Assuming you're looking for a fixed rate permanent loan on a stabilized property, I recommend regional & national banks.

I'd recommend passing on auctions. take your time, focus on land and land development and real estate development.

I would strongly recommend creating one to ensure everything is neatly organized and you don't get flagged by accidently writing off personal expenses. 

You will probably get other recommendations from other BP members. 

I paid the lot rent ($550)for 2 yrs on an empty house when I could have been collecting rentDid you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

For me personally I like mid-sized/regional bank as you are not just a number like with a national bank, but they still have some of the bells and whistles like good online tools that small banks lack.

Are there any resources (books, etc) you recommend to learn more in depth about this topic?

Are there calculators on this site that can calculate Net Present Value?